The present invention relates to a method and to a system for transmitting a binary data sequence in voice form from a piezoelectric transducer.
It is now common practice to use a memory card or xe2x80x9csmartxe2x80x9d card merely as a key for giving access to a service or as means for transferring information that can be confidential or otherwise.
In most intended applications, the access to a service or the transfer of information is preceded by executing an identification protocol of the unidirectional or bidirectional type which takes account at least of an identification sequence that is prerecorded in a memory of the card.
When access to a service is performed remotely, it is known to make use of xe2x80x9cvoicexe2x80x9d cards in which the identification sequence is converted into a sound signal which is transmitted over a telephone line via the microphone of the telephone handset.
In general, the identification sequence is transmitted via a piezoelectric transducer after being subjected to modulation of the frequency shift keying (FSK) type. Such modulation consists in emitting a carrier wave at a voice frequency that differs depending on the logic state of the bit to be transmitted. The term xe2x80x9cvoice frequencyxe2x80x9d is used to mean any frequency lying in the passband of the telephone network. That type of modulation is accepted as being the most suited for transmitting data by acoustic coupling with a telephone, because said modulation is relatively insensitive to noise or variation in the amplitude of the emitted signal.
The logic signal that results from FSK as applied to the piezoelectric transducer has a square waveform which is detrimental to good acoustic conversion. A squarewave signal generates numerous harmonics at levels that are almost as high as the level of the fundamental frequency. Such harmonics disturb transmission and are particularly disagreeable to the ear.
The object of the invention is to design a transmission method which mitigates the above-mentioned drawbacks while providing other advantages.
To this end, the invention provides a method of transmitting a binary data sequence in voice form from a piezoelectric transducer, the method being characterized in that it consists:
in defining a sinewave signal from a stream of pulses each having a logic xe2x80x9c0xe2x80x9d or a logic xe2x80x9c1xe2x80x9d state;
in generating a sampling frequency that differs depending on the logic state of the bit of the sequence that is to be transmitted;
in reading the stream of logic pulses at the sampling frequency corresponding to the logic state of the bit to be transmitted so as to obtain an intermediate logic signal;
in applying the intermediate logic signal alternately to one and then to the other of two input terminals of a piezoelectric transducer, at a voice frequency that differs depending on the logic state of the bit to be transmitted, so as to obtain a logic excitation signal made up of a positive half-cycle and of a negative half-cycle;
in reconstituting an acoustic signal having a sinusoidal waveform from the logic excitation signal by making use of the capacitive portion of the impedance of the piezoelectric transducer, in particular in the vicinity of its resonant frequency; and
in causing the transducer to emit the acoustic signal of sinusoidal waveform at the voice frequency which corresponds to the logic state of the bit to be transmitted.
In the method, the sinewave signal can be defined from a stream of i logic pulses corresponding to a positive or a negative half-cycle of the signal, by storing said stream of pulses in an i-bit rotary register, and by reading said register at the sampling frequency corresponding to the logic state of the bit to be transmitted.
In a variant, the sinewave signal can be defined from a stream of j logic pulses corresponding to a positive or a negative quarter-cycle of the signal, by recording said stream of pulses in a j-bit rotary register, and by reading said register from left to right and then from right to left at the sampling frequency corresponding to the logic state of the bit to be transmitted.
It is important to emphasize that the method has been designed to operate on logic signals only and using a single time base, thereby providing the particular advantage of being suitable for implementation using logic circuits that are simple, reliable, and of low cost.
Any analog solution would necessarily give rise to a physical embodiment that is complex, not-robust, and expensive.
The invention also provides a system for implementing the method, such a system being easily integrated in a voice type memory card of standard dimensions.